1. Field of the Invention
The present invention relates to a surface mount crystal oscillator, and more specifically, to a surface mount crystal oscillator and a substrate sheet each of which prevents a decrease in a frequency variable amount by reducing electrostatic capacitance of a crystal oscillator.
2. Description of the Related Art    [Prior Art]
A surface mount crystal oscillator is small in size and light in weight. In view of this, the surface mount crystal oscillator is provided, particularly, in portable electronic devices as a frequency or time reference standard.
Some conventional surface mount crystal oscillators are configured such that a crystal blank is mounted on a ceramic substrate and a cover having a recessed shape is provided upside down to cover the crystal blank so that the crystal blank is sealed and encapsulated therein. In recent years, there have been inexpensive oscillators for consumer use having a frequency deviation Δf/f which is relatively loosely regulated, for example, from ±150 to ±250 ppm.
In the conventional surface mount crystal oscillators, a crystal holding terminal for holding a crystal blank is formed on a front surface on which the crystal blank is mounted.
Further, on a rear surface of the surface mount crystal oscillator, patterns of ground (GND) terminals are formed at two portions generally.    [Related Art]
Note that, as related conventional techniques, there are Japanese Patent Application Laid-Open No. 04-217108 “Chip-type Piezoelectric Component” (Murata Manufacturing Co., Ltd.) [Patent Document 1], and Japanese Patent Application Laid-Open No. 04-296110 “Production Method of Piezoelectric Resonant Element” (Murata Manufacturing Co., Ltd.) [Patent Document 2].
Patent Document 1 discloses that an input electrode and an output electrode are provided so as not to face each other via an interposing substrate and they are placed with a considerable distance therebetween.
Patent Document 2 discloses that in a large-thickness portion of a piezoelectric substrate, those portions of electrodes which are not opposed to each other are made large in area, while in a small-thickness portion of the piezoelectric substrate, those portions of the electrodes which are not opposed to each other are made small in area, so that the volume of floating capacitance caused between the non-opposed portions of the electrodes is adjusted to restrain unevenness in resonant frequency.    [Patent Document 1] Japanese Patent Application Laid-Open No. 04-217108    [Patent Document 2] Japanese Patent Application Laid-Open No. 04-296110
However, the above conventional surface mount crystal oscillators have such a problem that a patterned portion of a crystal holding terminal formed on a front surface of a substrate and a patterned portion of a GND terminal formed on a rear surface of the substrate overlap each other across the substrate (a base material) and are opposed to each other, thereby causing an increase in electrostatic capacitance C0 of the crystal oscillator, which leads to a decrease in a frequency variable amount.
More specifically, the electrostatic capacitance is determined by the areas of metal plates, the distance between the metal plates, and the dielectric constant of an insulator provided between the metal plates.
Further, as for the crystal oscillator, a larger frequency variable amount may be required by clients in some cases. In this regard, since the frequency variable amount ΔF/F is given in accordance with ΔF/F=C1/2(CL+C0), the increase in the electrostatic capacitance C0 will lead to the decrease in the frequency variable amount.